drm/i915: Reduce locking inside swfinish ioctl

We only need to take the struct_mutex if the object is pinned to the
display engine and so requires checking for clflush. (The race with
userspace pinning the object to a framebuffer is irrelevant.)

v2: Use access once for compiler hints (or not as it is a bitfield)
v3: READ_ONCE, obj->pin_display is not a bitfield anymore
v4: Don't be creative with goto.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470388464-28458-14-git-send-email-chris@chris-wilson.co.uk

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 1d8858d7973d33b4789eb991406aa48db453e572..03e806ce75ea856f115ac06cfaf1dec8d88c319b 100644 (file)
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1518,26 +1518,23 @@ i915_gem_sw_finish_ioctl(struct drm_device *dev, void *data,
 {
        struct drm_i915_gem_sw_finish *args = data;
        struct drm_i915_gem_object *obj;
-       int ret = 0;
-
-       ret = i915_mutex_lock_interruptible(dev);
-       if (ret)
-               return ret;
+       int err = 0;
 
        obj = i915_gem_object_lookup(file, args->handle);
-       if (!obj) {
-               ret = -ENOENT;
-               goto unlock;
-       }
+       if (!obj)
+               return -ENOENT;
 
        /* Pinned buffers may be scanout, so flush the cache */
-       if (obj->pin_display)
-               i915_gem_object_flush_cpu_write_domain(obj);
+       if (READ_ONCE(obj->pin_display)) {
+               err = i915_mutex_lock_interruptible(dev);
+               if (!err) {
+                       i915_gem_object_flush_cpu_write_domain(obj);
+                       mutex_unlock(&dev->struct_mutex);
+               }
+       }
 
-       i915_gem_object_put(obj);
-unlock:
-       mutex_unlock(&dev->struct_mutex);
-       return ret;
+       i915_gem_object_put_unlocked(obj);
+       return err;
 }
 
 /**